In radio transmitters for broadcast, cellular and satellite systems, the power amplifier (PA) in the transmitter has to be very linear, in addition to being able to simultaneously amplify many radio channels (frequencies) or independent user data channels, spread across a fairly wide bandwidth. High linearity is required since nonlinear amplifiers would cause leakage of interfering signal energy between channels and distortion within each channel.
Two methods have been widely used in Radio Frequency (RF) applications for overcoming the low efficiency of conventional linear power amplifiers, namely the Chireix outphasing method and the Doherty method, further details of which can be found in “High Power Outphasing Modulation” by H Chireix, Proc. IRE, vol 23, no 2, pp 1370-1392, November 1935 and “A New High Efficiency Power Amplifier for Modulated Waves” by W. H. Doherty, Proc. IRE, vol. 24, no. 9, pp 1163-1182, September 1936. Chireix and Doherty amplifiers were the first examples of RF amplifiers based on multiple transistors with passive output network interaction and combination that gave high average efficiency for amplitude-modulated signals.
The outphasing technique in Chireix amplifiers involves combining several (generally two) phase-modulated constant-amplitude signals. These signals are produced in a “signal component separator” (SCS) and subsequently, after up-conversion and amplification through RF chains (comprising mixers, filters and amplifiers), combined to form an amplified linear signal in an output combiner network. The phases of these constant-amplitude signals are chosen so that the result from their vector summation yields the desired amplitude. An advantage of a Chireix amplifier is its ability to change an efficiency curve to suit different peak-to-average power ratios.
Amplifiers based on passive output network interaction structures have the advantage of needing only fundamental radio frequency (RF) network and signal modifications. Compared to single-transistor amplifiers they differ only in the number of independently driven transistors. Harmonics and/or baseband modifications, that are required for other high-efficiency amplifiers, are optional.
The field has been generalized for two-transistor structures, for example as disclosed in U.S. Pat. No. 6,940,349 by the present Applicant. Examples of expandable multi-transistor structures, and ways to drive them efficiently, are disclosed in U.S. Pat. Nos. 7,279,971 and 7,411,449 by the present Applicant. These structures are sufficient to provide all combinations of Doherty and Chireix efficiency curve features.
Recently, an alternative way to construct multi-transistor Chireix amplifiers with 4, 8, 16, (powers of two) number of differently driven transistors were proposed by Perreault et al, in a paper entitled “A New Power Combining and Outphasing Modulation System for High-Efficiency Power Amplification”, IEEE Proc. MWSCAS 2010, pages 441-444. These amplifiers have the properties that all constituent transistors have equal size and operate very similarly. The amplifiers of this type operate in an “outphasing” mode, i.e. maximum voltage operation when only the phase difference between transistor output voltages vary, over a wide range of output amplitudes. In this range, the sum of the output RF current amplitudes varies almost parabolically with output amplitude. This wide range of outphasing is close to optimal for transistors and frequencies for which series losses dominate even at low output powers. The structure of these amplifiers is shown in FIGS. 1a and 1b. 
FIG. 1a shows the structure of a “two-level” amplifier as proposed by Perreault et al., which comprises four sources 1011 to 1014 (for example transistors) and the output network arranged in two levels (Level 1 and Level 2).
FIG. 1b shows the structure of a “three-level” amplifier having eight sources 1011 to 1018 (for example transistors), with the output network arranged in three levels, Level 1, Level 2 and Level 3.
The reactances denoted +/−jX are capacitors and inductors. The same electrical behavior can also be obtained with different lengths of transmission line instead of reactances, as disclosed in U.S. Pat. Nos. 7,279,971 and 7,411,449.
In the paper by Perreault it is specifically stated that four or more transistors or sources are used, where “or more” means higher powers of two (that is 8, 16, 32, 64, etc.).
The extra complexity and power consumption for driving four transistors can, in many cases, be so high that it negates the efficiency benefit of the four-transistor amplifier itself.
Similar problems occur for the higher powers of two, whereby the extra efficiency benefit of an 8-transistor Chireix amplifier, for example, may be outweighed by the additional complexity.
The inventions described in U.S. Pat. Nos. 7,279,971 and 7,411,449 have the ability to form amplifiers with any number (larger than two) of differently driven transistors. However, for the specific situation for which the amplifiers in Perreault are the closest to optimal (transistors and frequencies for which series losses dominate even in very low output power ranges) these amplifiers are less close to optimal.